Experimental studies on the characteristics of evaporative heat transfer and pressure drop of CO2/propane mixtures in horizontal and vertical smooth and micro-fin tubes

This study presents evaporative heat transfer characteristics of CO2/propane refrigerant mixtures in horizontal, and vertical smooth and micro-fin tubes. The effect of mass flux, heat flux, inlet temperature and several compositions have been investigated and analyzed. The copper tube with outer diameter of 5 mm and length of 1.44 m was selected as the test sections. Average inner diameters of test tubes are 4.0 mm and 4.13 mm, respectively. The tests were conducted at mass fluxes from 212 to 656 kg/m2, heat fluxes from 15 to 60 kW/m2, inlet temperatures from −10 to 30 °C, and for several compositions (75/25, 50/50, 25/75 wt%). Among CO2/propane refrigerant mixtures, the heat transfer characteristics are much better than those of any other compositions when the composition is 75/25 (wt%). Finally, the heat transfer coefficients of CO2/propane refrigerant mixtures in the vertical tube are 5–10% higher than those in horizontal tubes.